European Patent No. 0 523 374 describes a superconductive current-limiting device.
In a.c. power supply systems it is impossible to reliably prevent short-circuits and arcing, which cause the alternating current to increase very rapidly in the respective circuit, i.e., to a multiple of its rated value in the first half-wave until it is interrupted by suitable fuse means or circuit-breaker means. As a result, great thermal and mechanical stresses occur in all the components of the system affected, such as conductors and busbars, switches or transformers, due to electromechanical forces. Since these transient loads increase with the square of the current, reliably limiting the short-circuit current to a lower peak level can greatly reduce the requirements regarding the load capacity of these system components. This can result in cost advantages, e.g., in constructing new systems or in expanding existing systems since it is possible to avoid replacing system components with higher-rated designs by installing current-limiting devices when they are rising short-circuit currents.
With superconducting current-limiting devices of the resistive type, the current rise after a short-circuit can be limited to a value of a few multiples of the rated current. Also, the current-limiting device is ready for operation again a short time after shutdown. The device acts like a fast, self-healing fuse. The device ensures high operational reliability because it is passive, i.e., it operates autonomously without prior detection of the short-circuit and active triggering by a switch signal.
Resistive superconducting current-limiting devices form a superconducting breaker gap that is to be inserted into a circuit in series. The devices utilize the transition of a superconducting printed conductor from the cold, practically zero-resistance operating state below the critical temperature T.sub.c of the superconductor material to the normal-conducting state above T.sub.c, with the electric resistance R.sub.n which then prevails in the printed conductor limiting the current to an acceptable level I=U/R.sub.n. Heating above the critical temperature T.sub.c is accomplished by Joulean heat in the superconductor of the printed conductor itself, when the current density j rises above the critical value j.sub.c of the superconductor material after a short circuit, with the material already having a finite electric resistance even below the critical temperature T.sub.c. In the limiting state above the critical temperature T.sub.c a residual current continues to flow in the circuit until an additional mechanical disconnect switch completely interrupts the circuit.
Superconducting current-limiting devices with known metal oxide high-T.sub.c superconductor materials whose critical temperature T.sub.c is so high that they can be kept in the superconducting operating state with liquid nitrogen at 77 K show a rapid increase in electric resistance on exceeding the critical current density j.sub.c. Heating to the normal-conducting, i.e., current-limiting state takes place in a relatively short period of time, so the peak value of the short-circuit current can be limited to approximately three to ten times the rated current, a fraction of the unlimited current. The superconducting current path is in contact with a coolant capable of returning it to the superconducting operating state in a relatively short period of time after exceeding the critical current density j.sub.c.
Corresponding requirements can be largely met with the current-limiting device derived from the European Patent No. 0 523 374. The known device contains a meandering electric conductor of a HTSL material made out of a 5 mm-thick sheet of HTSL material by slotting. The electric conductor is self-supporting, but it may be arranged on a carrier body to increase its mechanical stability. The HTSL material is produced by powder metallurgy and has a relatively low critical current density j.sub.c so that the cross section of the printed conductor must be large enough for rated currents of 25 A, for example. A large cross section also necessitates a great length to produce the normal conduction resistance required for the limiting effect. Large amounts of HTSL material are therefore required. A large amount of material also means long cooling times after the limiter responds (switching operation); i.e., the current can be turned on again only after a considerable delay.